The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Switching From Mounjaro To Wegovy — What You Need To Know
If you're considering swapping from Mounjaro to Wegovy, you're comparing two different weight loss injection mechanisms and clinical profiles. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1-only medication. Both are prescription-only and require clinical assessment. Switching between them isn't straightforward — your prescriber needs to manage the transition carefully, as they work differently and have different dose strengths. This guide explains the key differences to help you understand what your GP or prescriber considers when evaluating a switch.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Mounjaro And Wegovy Differ — Mechanism And Clinical Data
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways, which is why it has a dual mechanism. In the SURMOUNT-1 clinical trial, participants on 15 mg achieved an average weight loss of approximately 20.9% of body weight over 72 weeks. Wegovy (semaglutide) activates the GLP-1 pathway only. In the STEP-1 trial, the 2.4 mg dose resulted in an average weight loss of approximately 14.9% over 68 weeks.
This difference in mechanism means Mounjaro and Wegovy work in slightly different ways. Some people respond better to one than the other, though individual results vary significantly. Mounjaro typically shows numerically higher average weight loss in clinical trials, but this does not mean it's "better" — suitability depends on your individual health profile, side effect tolerance, and how your body responds.
- Mounjaro: dual GIP + GLP-1 agonist, doses 2.5–15 mg weekly
- Wegovy: GLP-1 agonist only, doses 0.25–2.4 mg weekly
- Both available as once-weekly injections in the UK
- Both require prescription and clinical assessment
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Why People Consider Switching — And What Your Prescriber Will Assess
Reasons for considering a switch include insufficient weight loss on your current dose, troublesome side effects (nausea, constipation, or diarrhoea), or tolerability concerns. Your prescriber will assess whether a medication switch makes clinical sense for you specifically.
There is no official established conversion chart between Mounjaro and Wegovy doses — approximate equivalences exist (5 mg Mounjaro ≈ 0.5 mg Wegovy, 10 mg ≈ 1.7 mg, 15 mg ≈ 2.4 mg), but any actual switch must be managed by your prescriber. You cannot simply stop one and start the other without guidance. Your GP or weight management specialist will consider your current response, your medical history, and whether a medication change is justified. Use our free GLP-1 weight loss calculator to explore projected outcomes and discuss options with your prescriber.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Can I switch from Mounjaro to Wegovy without my prescriber's permission?
No. Both Mounjaro and Wegovy are prescription-only medications. Switching between them requires your prescriber's approval and supervised management. Your GP or specialist will assess whether a change is clinically appropriate for you based on your individual response and health profile.
Will I lose the same amount of weight on Wegovy as I did on Mounjaro?
Not necessarily. Clinical trial data shows different average outcomes (Mounjaro ~21%, Wegovy ~15%), but individual results vary significantly. Some people lose more on Wegovy, others on Mounjaro. Your weight loss depends on many factors including diet, activity, genetics, and how your body responds to each medication.
What happens to my doses if I switch from Mounjaro to Wegovy?
Your prescriber will not simply convert your Mounjaro dose directly. Wegovy has different dose strengths and titration schedules. You'll typically restart on a lower Wegovy dose (0.25 mg) and gradually increase, even if you were on a higher Mounjaro dose. This is because the medications work through different mechanisms.
Is Wegovy better than Mounjaro for weight loss?
Neither is objectively "better" — they work differently. Mounjaro shows numerically higher average weight loss in trials, but individual suitability varies. Some people respond better to one, others to the other. Your prescriber will recommend based on your health, side effects, and individual response — not on trial averages alone.
How long does it take to see results after switching to Wegovy?
Weight loss typically becomes noticeable around week 4–8 as your dose increases. However, if you've recently stopped Mounjaro, there may be a transition period. Your prescriber will advise on timing and expectations. Track your progress using our free progress tracker alongside your prescriber's guidance.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.